The invention relates to a method of manufacturing optical fibers. In the method layers of glass are deposited on the inner wall of a glass tube by heating the tube to a temperature between 110.degree. and 1300.degree. C., by passing a reactive gas mixture through a glass tube at the pressure between 1 and 30 hectopascal (hPa) in the interior of the glass tube a plasma by forming is reciprocated strokewise between two reversal points. After a sufficient amount of glass (corresponding to the intended optical fiber construction) has been deposited, the glass tube is then collapsed to form a solid preform from which optical fibers are drawn.
"Glass tube" is to be understood to mean in this connection a substrate tube or coating tube which consists either of (i) synthetically made amorphous silica to (ii) amorphous silica made by melting quartz crystals (fused silica or quartz glass), or (iii) both synthetically made amorphous silica and amorphous silica made by melting quartz crystals. The silica making up the glass tube may optionally be doped.
By means of the above-mentioned method, both graded index fibers and step index fibers can be manufactured. The glass are deposited in a manner corresponding to the intended fiber construction.
The manufacture of optical fibers or optical wave guides, according to the above-mentioned method, is described for example in U.S. Pat. Nos. Re 30,635 and 4,314,833. Commonly, this method is referred to as the "non-isothermal plasma CVD method" (non-isothermal PCVD method, where P=plasma and CVD=chemical vapor deposition =reactive deposition from a gas phase). In this method, glass layers are deposited directly from the gas phase on the inner wall of the glass tube (a heterogeneous reaction). The formation of glass soot in the gas phase is avoided. This is described in greater detail in particular in U.S. Pat. No. 4,314,833.
The essential difference between the PCVD method and the MCVD method, Modified CVD method (which is also used for the manufacture of optical fibers by coating the inside of a tube), resides in the manner in which the chemical reactions necessary for the deposition of glass components are activated. The MCVD method is based on thermal activation by a burner; the PCVD method uses activation by electron impact excitations. In the MCVD method, in a first stage fine dust particles (glass soot) are formed. Deposition of the soot in the temperature and gravity fields along the tube will be uniform over an extensive area and over the circumference of the tube only when the tube is rotated. Compact glass layers are obtained sintering the soot after deposition. In contrast, fine dust particles are not formed in the electron impact excitation of the PCVD method. The gaseous reaction products are rather present in a molecular form and can reach the inner wall of the glass tube via comparatively rapid diffusion and condense thereon. Consequently, the deposition is narrowly, localized and uniform over the circumference of the tube. Sintering of the deposited material is not required for obtaining a vitreous layer. This is also the reason why in the PCVD method glass can be deposited in both directions of movement of the plasma. In the MCVD method glass particles can be deposited only in one direction, namely in the direction of flow of the reactive gases.
The efficiency of the described technological methods of manufacturing preforms for optical fibers is determined decisively by the yields to be achieved by the specific methods. Essential parameters in this respect are the reproducibility of the method, the chemical reaction yields, the deposition rate and the optical and geometrical homogeneity of the deposited material over the length of the preform.
As a result of the specific reaction and deposition mechanisms, the PCVD method permits the manufacture of optically high-grade preforms with high reproducibility, reaction yields near 100% and, in comparison with other methods, low entrance taper losses. Entrance taper is to be understood to mean herein a deposition range with not sufficiently constant optical and geometrical properties at the entrance of the preform (i.e. on the gas inlet side). The homogeneous range between the entrance taper and a corresponding range at the preform end is hereinafter referred to s the "plateau region".
As transport to the tube wall in the PCVD method is determined by a rapid molecular diffusion mechanism, the extension of the deposition in the longitudinal direction of the substrate tube is, of course, very small. Homogeneous plateau regions are obtained by reciprocating the reaction zone at constant speed in the longitudinal direction of the tube. The regions of non-constant velocity at the preform ends, required for the reversal of the plasma, are chosen to be as small as possible.
With deposition rates of approximately 0.5 g/minute and preform lengths of approximately 70 cm these conditions in the PCVD method result in relatively low yield losses by taper approximately 15% (i.e. a taper length of approximately 10 cm). For practical reasons the taper length is defined as the distance between the locations where the amount of deposited glass is 10 and 90%, respectively, of the maximum amount of deposited glass in the plateau region.
Due to the different reaction and transport mechanisms as compared to the PCVD method, completely different deposition conditions are present with the MCVD method. In particular, due to the formation of glass soot by a homogeneous gas phase reaction, deposition distributions extend in the longitudinal direction of the tube and hence make additional measures for the reduction of taper losses necessary.
A measure for taper reduction in the MCVD method is described in U.K. Patent Application 2,181,165. In this document, taper is reduced by varying the speed of the burner along the tube (i.e. by moving the burner nonlinearly in a mechanical way. The nature of the movement is derived from a specific deposition function which in the MCVD method depends in a complex manner on all deposition parameters. This function must be determined experimentally by an iteration method for the specific deposition conditions. In all cases a special nonlinear movement over the whole stroke length is require. "Stroke length" is the be understood to mean the length of travel of the burner between the reversal points.
Although other possible methods of taper reduction, (for example flow variations or linear mechanical ramps, which are to be understood to mean a reduction of the burner speed at the gas inlet side) are discussed, they are also considered to be impractical or insufficient in their action. The reason for this--and hence for the above-mentioned special measures--is to be found in the MCVD method itself and is due to the following specific properties of the MCVD:
The particle size distribution and the building-in behavior for SiO.sub.2 and dopants in the MCVD method depends in an extremely complex manner on all method parameters due to the thermal reaction with glass soot formation. Consequently, the deposition function can be determined only experimentally--but cannot be calculated in a quantitative way from theory.
The glass soot formation which is accompanied by deposition yields for SiO.sub.2 which are below 100%, leads to an extension of the deposition region in the MCVD method. The deposition region larger than the stroke length along the tube) (i.e. glass soot also escapes in a rather large quantity from the end of the tube. For this reason to obtain a somewhat constant layer thickness, the burner must be moved nonlinearly over the whole stroke length. A homogeneous plateau region actually does not exist in the MCVD method.
A variation of process parameters other than the burner velocity for taper reduction immediately varies (again as a result of the complex behavior of building-in, yield and dust particles size the deposition profile and deposition function, the dopant incorporation, and the yields--and that simultaneously. As a result, such variations are difficult to control in the MCVD method and cannot be carried out without negative effects on the optical quality and homogeneity of the deposit.
As may be understood from what is said about the prior art, the PCVD method presents specific advantages with respect to taper losses. These advantages of the PCVD method render the expensive and problem-causing measures for taper reduction and for achieving homogeneous plateau regions in MCVD preforms redundant.
However, it is still desirable in the PCVD method to further reduce taper losses and hence to further increase process yields. In particular it should be an object to keep relative taper losses as small as possible (i.e. typically &lt;10% of the preform length) in case the deposition rates are increased to values greater than 0.5 g/min.